Remember when you could earn TWA miles on (now defunct) Eastern Airlines and Eastern miles on TWA? It was the mid- to late-1980's, I believe.

You were only supposed to collect for one airline or the other, not both. For TWA miles you stuck a sticker on the ticket with your account number on it. For Eastern miles you handed in a piece of paper with your Eastern account number and flight number on it. Bottom line: If you had an ignorant or cooperative gate attendant, you could hand in the piece of paper AND a sticker, and illegally get mileage on both airlines for the same flight. This petty larceny used to give me a lot of pleasure, particularly because of the passenger abuse Eastern dished out.